Combined inductive sensing and capacitive sensing

ABSTRACT

A sense unit for inductive sensing or capacitive sensing is described. The sense unit may include a first terminal coupled to a first node, a first electrode coupled to the first node, and a second terminal. The sense unit may include a second electrode coupled to the second terminal. In a first mode, a first signal is received at the first terminal and a second signal is output on the second terminal, where the second signal may be representative of a capacitance of the sense unit. The sense unit may include an inductive coil. The sense unit may include a first capacitor. The inductive coil and the first capacitor are coupled in parallel between the first node and ground. In a second mode, a third signal is received at the first terminal and a fourth signal is output on the second terminal.

RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.16/391,069, filed on Apr. 22, 2019, which is a Continuation of U.S.patent application Ser. No. 15/637,731, filed on Jun. 29, 2017, now U.S.Pat. No. 10,444,916, issued Oct. 15, 2019, which claims the benefit ofU.S. Provisional Application No. 62/470,061, filed on Mar. 10, 2017, andU.S. Provisional Application No. 62/470,044, filed on Mar. 10, 2017, allof which are incorporated by reference herein in their entirety.

BACKGROUND

A touch sensor may be used to detect the presence and location of anobject or the proximity of an object within a touch-sensitive area ofthe touch sensor. For example, touch sensing circuitry may detect thepresence and location of a touch object proximate to a touch sensordisposed in connection with a display screen. There are a number ofdifferent types of touch sensors. The types of touch sensor may includeresistive touch sensors, surface acoustic wave touch sensors, capacitivetouch sensors, inductive touch sensing, and so forth. The differenttouch sensors may detect different types of objects.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is illustrated by way of example, and not oflimitation, in the figures of the accompanying drawings in which:

FIG. 1 illustrates sensing circuit for self-capacitance sensingaccording to an embodiment.

FIG. 2A illustrates device for capacitive sensing and inductive sensing,according to an embodiment.

FIG. 2B illustrates the sensing unit that operates as a variablecapacitor in a capacitance mode when a frequency of the TX signal isbelow or above a resonant frequency, according to one embodiment.

FIG. 2C illustrates the sensing unit that operates as a variableinductor in an inductance mode when the phase of the TX signal has beenshifted to excite a resonant circuit, according to one embodiment.

FIG. 2D illustrates a circuit level diagram of the device performingfull-wave capacitive sensing and inductive sensing, according to anembodiment.

FIG. 2E illustrates the charge measurement circuit (CMC) of the devicethat is configured for half-wave capacitive sensing and inductivesensing according to an embodiment.

FIG. 3 illustrates a sense unit where the resonant circuit includes aresistor, an inductor, a capacitor, a second capacitor, and a ground,according to an embodiment.

FIG. 4A illustrates the sense unit that includes a capacitor and aninductor, according to an embodiment.

FIG. 4B illustrates the sense unit of FIG. 4A that includes a flat coil,according to an embodiment.

FIG. 4C illustrates the sense unit of FIG. 4A that includes a flat coilwith a small inner circumference, according to an embodiment.

FIG. 4D illustrates the sense unit of FIG. 4A that includes a rectanglecoil, according to an embodiment.

FIG. 4E illustrates the sensing unit of FIG. 4A that includes a multiplelayer coil, according to an embodiment.

FIG. 4F illustrates the sense unit of FIG. 4A that includes a primarycoil and a secondary coil, according to an embodiment.

FIG. 4G illustrates the sense unit of FIG. 4A that includes the primarycoil 438 and the secondary coil that are coplanar, according to anembodiment.

FIG. 5 illustrates a sensing circuit with a hybrid capacitive andinductive sensor, according to an embodiment.

FIG. 6A illustrates the sense unit of FIG. 5 that includes the firstcircuitry with a capacitor and the second circuitry with an inductor,according to an embodiment.

FIG. 6B illustrates the sense unit of FIG. 5 that includes the firstcircuitry with the capacitor and the second circuitry with the inductor,a capacitor, and a ground, according to an embodiment.

FIG. 6C illustrates the sense unit where the GPIO is open and the GPIOis closed, according to an embodiment.

FIG. 6D illustrates the sense unit of FIG. 5 that includes a firstcircuitry with a capacitor and the second circuitry with the inductor, acapacitor, an inductor, a capacitor, and a ground, according to anembodiment.

FIG. 7 illustrates a graph of an amplitude change associated with adigital representation of the RX signal, according to an embodiment.

FIG. 8A illustrates a graph of a phase shifting and demodulation of a TXsignal for inductive sensing, according to an embodiment.

FIG. 8B illustrates a graph with a TX signal for capacitive sensing,according to an embodiment.

FIG. 9 illustrates a graph showing the phase shifting and demodulationof a resonant circuit output signal for inductive sensing, according toan embodiment.

FIG. 10A shows a graph of the frequencies used by the CMC in FIG. 2A forinductive sensing, according to an embodiment.

FIG. 10B shows a graph another frequency that may be used by the CMC inFIG. 2A for inductive sensing, according to an embodiment.

FIG. 10C shows a graph another frequency used by the CMC in FIG. 2A forinductive sensing, according to an embodiment.

FIG. 11 illustrates a flow diagram of a method of determining aninductance of a sensing unit, according to another embodiment.

FIG. 12 illustrates a flow diagram of a method of applying signals to afirst electrode and a second electrode, according to another embodiment.

DETAILED DESCRIPTION

Many electronic devices include touch sensors (also referred to hereinas sense units or unit cells) for the user to interact with theelectronic devices. For example, automatic teller machines (ATMs),information kiosks, smartphones, vending machines, washing machines,televisions, computers, and refrigerators may include sense units andcorresponding touch sensing circuitry. When an object touches or isproximate to the sense unit, the touch sensing circuitry may be used tocapture and record the presence and location of the objects using thesense unit.

Unlike buttons or other mechanical controls, sense units may be moresensitive and may respond differently to different types of touch, suchas tapping, swiping and pinching. The different sense units may alsorespond differently to different types of objects. There are varioustechniques for measuring capacitance, inductance, or resistance, butthese different techniques use different types of sense units anddifferent circuits to measure capacitance, inductance, or resistance.For example, the inductive sensing may be used to detect ferrous andnon-ferrous metals and the capacitive sensing may be used to detectferrous and non-ferrous conductive objects. Conventionally, to detectthe different types of objects, a device would have to include differentsense elements and different circuits to measure these different typesof objects. Integration of these different sense elements and circuitrymay not be feasible in terms of cost or available space within thedevice, especially when the device form factor is small.

Embodiments of the present disclosure describe technology for combinedinductive and capacitive sensing. The embodiments may provide a senseunit that may be used to detect different types of objects and combinedinductive and capacitive sensing circuitry that may be used to detectthese different types of object using inductive sensing and capacitivesensing. In one embodiment, the sense unit may be used for capacitivesensing in a first mode and the same sensing unit can be used forinductive sensing in a second mode. In one embodiment, the combinedinductive and capacitive sensing circuitry (also referred to herein as“touch sensing circuitry” or “sensing circuitry”) using a type ofcapacitive sensing circuitry in a way that it can measure inductance ofthe sense element in a first mode (inductive sensing mode) andcapacitive of the sense element in a second mode (capacitive sensingmode), as described in more detail herein. When the sensing circuitoperates in the capacitive sensing mode, the sensing circuit may use thesense unit to detect objects using a capacitive sensing technique. Whenthe sensing circuit operates in the inductive sensing mode, the sensingcircuit may detect ferrous and non-ferrous metal objects proximate tothe sense unit using inductive sensing techniques.

In one embodiment, the sensing circuitry includes 1) a signal generatorto output on a first terminal a first signal in a first mode and asecond signal in a second mode; and 2) a charge measuring circuit toreceive on a second terminal a third signal in the first mode and afourth signal in the second mode. The third signal is representative ofan inductance of a sense unit coupled between the first terminal and thesecond terminal and the fourth signal is representative of a capacitanceof the sense unit.

In another embodiment, a sense unit includes a first terminal coupled toa first node, a first electrode coupled to the first node, a secondterminal, and a second electrode coupled to the second terminal. Thesense unit also includes an inductive coil and a first capacitor. In afirst mode (capacitive sensing mode), a first signal is received at thefirst terminal and a second signal is output on the second terminal. Thesecond signal is representative of a capacitance of the sense unit. In asecond mode (inductive sensing mode), a third signal is received at thefirst terminal and a fourth signal is output from the second terminal.The fourth signal is representative of an inductance of the sense unit.

FIG. 1 illustrates sensing circuit 100 for self-capacitance sensingaccording to an embodiment. The sensing circuit 100 may include aprocessor 119, a charge measurement circuit (CMC) 110, a general purposeinput/output (GPIO) 112, a sense capacitor (C_(sense)) 114, a GPIO 116,and a modulator capacitor (C_(mod)) 118. The GPIO 112 and GPIO 116 maybe any types of terminals that are configured to couple to externalcomponents, such as the sense units, as well as other external devices.In one example, GPIOs may be terminals that are points of connection toa circuit. The GPIOs that may couple to pins, pads, solder bumps, or thelike. In another example, GPIOs may include specialized outputs,dedicated outputs/inputs, or the like. The GPIOs may be internal routingmechanism to connect pins or pad to power source, a ground, a high-Z,internal circuitry (such as sensing circuitry), a pulse width modulator(PWM), or the like.

In FIG. 1 , GPIO 112 is coupled to a sense element 114, which is asingle electrode. The CMC 110 can measure a self-capacitance of thesingle electrode with respect to a ground potential. As such, the senseelement 114 (C_(sense)) is represented as an external capacitor. The CMC110 may use a modulator capacitor (C_(mod)) 118 for measuring thecapacitance on sense element 114. In some embodiments, the C_(mod) 118is an external capacitor coupled to GPIO 116, as illustrated in FIG. 1 .In one example, the CMC 110 may be a capacitance-to-digital converter(CDC). In another example, the CMC 110 may be a charge transfer circuit,a capacitive sensing charge measurement circuit, a capacitive sensingsigma-delta (CSD) circuit, or the like. The CSD circuit may includephysical, electrical, and software components.

The CSD circuit may have an array of capacitive sense elements (as thesense units) coupled to a sigma-delta modulator though an analogmultiplexer, digital counting functions, and high-level softwareroutines to compensate for environmental and physical sensor elementvariations. The physical component may include the physical sensorelement itself, typically a pattern constructed on a printed circuitboard (PCB) with an insulating cover, a flexible membrane, or atransparent overlay. In one embodiment, the physical component may alsoinclude a transparent conductor, such as indium tin oxide (ITO) disposedon a substrate, which may be transparent as well. The electricalcomponent may convert a charged capacitance into a measured signal. Theelectrical component may include an operational amplifier that mayoutput a bit stream that may be quantified by a counter circuit or atimer circuit. The software component may include detection andcompensation software algorithms to convert a count value into a sensorelement detection decision.

As illustrated in FIG. 1 , the CMC 110 may be coupled to the GPIO 112and the GPIO 116. In one example, the GPIO 112 may be a first terminalof the CMC 110 and the GPIO 116 may be a second terminal of the CMC 110.Terminals may be pins, pads, solder bumps, or other mechanisms toconnect conductors of the different devices or components. Duringoperation, the CMC 110 may use the sense element 114 (C_(sense)) to takea charge measurement. For example, the CMC 110 may charge and dischargethe electrode of the sense unit in order to measure a capacitance of thesense element 114 (C_(sense)). The CMC 110 may digitize the measuredcapacitance into an accumulated voltage value or bit stream. In oneembodiment, the CMC 110 may compare the charge measurement to areference value to determine a difference between the charge measurementand the reference value. In one embodiment, the reference value may be aprevious value measured by the charge measurement circuit 110. Inanother embodiment, the reference value may be a predefined value. Thepredefined value may be a default reference value or derived frompreviously measured charge measurement values taken over time. Thedifference value may indicate a difference between the chargemeasurement and reference value. In one example, the CMC 110 may sendthe difference value to the processor 119. The processor 119 or the CMC110 may determine whether the difference value exceeds a thresholdvalue. When the difference value exceeds the threshold value, thedifference value indicates that an object is proximate to the senseelement 114 (C_(sense)). When the difference value does not exceed thethreshold value, the difference value indicates that no object that maybe detected using capacitive sensing is proximate to the C_(sense) 114.

FIG. 2A illustrates device 200 for capacitive sensing and inductivesensing, according to an embodiment. The device 200 can measure acapacitance of a sense unit 225, an inductance of the sense unit 225, orboth, as described herein. The device 200 includes a charge measurementcircuit 210 that is coupled to the sense unit 225. In a capacitivesensing mode, the charge measurement circuit 210 may include a signalgenerator 229 that can generate an excitation (TX) signal that can beapplied via GPIO 226 to the sense unit 225 and a receiver channel (notillustrated in FIG. 2A) that can measure a receive (RX) signal at GPIO220. The RX signal is representative of a mutual capacitance of thesense unit 225. In another embodiment, the charge measurement circuit210 can charge and discharge the sense unit 225 via GPIO 220 to measurea self-capacitance.

In the inductive sensing mode, the charge measurement circuit 210 mayphase shift the TX signal relative to a reference signal to generate aphase-shifted signal to be applied to the sense unit 225. For example,the charge measurement circuit 210 can output the TX signal to a smartinput/output interface 230 (herein after “smart IO 230”) and a pulsewidth modulator (PWM) 228 to generate a phase-shifted TX signal (e.g.,TX signal shifted by 90 degrees) as described herein. In anotherexample, the smart IO 230 and/or the PWM 228 may shift a phase of asignal. A second receive (RX) signal can be measured by the chargemeasurement circuit 210 via GPIO 220. The second RX signal isrepresentative of an inductance of the sense unit 225.

The charge measurement circuit 210 can use a modulator capacitor(C_(mod)) 214 and another capacitor (C_(Tank)) 218 in capacitive sensingmode, inductive sensing mode, or both, as described herein. The chargemeasurement circuit 210 may also include an analog-to-digital converter(ADC) to convert the analog measurements of the capacitance andinductance of the sense unit 225 into digital values. These digitalvalues can be output to a processor 219 for further digital signalprocessing for an application.

As illustrated in FIG. 2A, the sense unit 225 may have a resonantcircuit 224 and a pair of electrodes (a first electrode and a secondelectrode) that form a sense capacitor 222. Various embodiments of thesense unit 225, including the resonant circuit 224 are described andillustrated with respect to FIGS. 2B-6D. In the capacitive sensing mode,the CMC 210 may measure a mutual capacitance of the sense unit 225,where the mutual capacitance is between the first electrode and thesecond electrode of sense capacitor 222. In the inductive sensing mode,the CMC 210 may measure an inductance for inductive sensing, asdescribed herein. For example, the CMC 210 may measure an inductance ofa sense unit 225. The number of sense units 225 that the device 200includes is not intended to be limiting. For example, the device 200 mayinclude a single sense unit 225 or multiple sense units 225.

In one embodiment, the processor 219, the CMC 210, the C_(mod) 214, theC_(Tank) 218, the PWM 228, and/or the smart IO 230 of the device 200 maybe located on the same integrated circuit having the GPIO 226, 212, 220,216 that are coupled to the sense unit 225, the modulator capacitor(C_(mod)) 214, and the tank capacitor (C_(tank)) 218. Alternatively, thedifferent components of the device 200 may be implemented in multipleintegrated circuits. The modulator capacitor (C_(mod)) 214 and the tankcapacitor (C_(tank)) 218 may be located, at least in part, external tothe integrated circuit containing the charge measurement circuit 210.

As noted above, the CMC 210 may include a signal generator 229. Thesignal generator 229 may generate a transmit (TX) signal (also referredto as an excitation signal). In one example, the TX signal may be asquare wave signal. The smart IO 230 may be coupled to the CMC 210. Thesmart IO 230 may be a digital controller of an IO interface that isexternal to a microcontroller (MCU) or not coupled to the MCU. The smartIO 230 may offload tasks from the MCU to provide a configuration of IOs.

In one embodiment, the PWM 228 may be coupled to the smart IO 230. Inanother embodiment, the PWM 228 may be coupled directly to the CMC 210.The PWM 228 may adjust a phase of the TX signal. In one example, aninput of the PMW 228 may be mixed with the TX signal to adjust a phaseof the TX signal.

A phase of the TX signal may be shifted as the device 200 takes aninductive sensing measurement. Alternatively, there may be otherembodiments where the TX signal is shifted for capacitive sensingmeasurement. In one example, the signal generator 229 may generate a TXsignal with a phase of 0 degrees relative to a reference signal. Whenthe phase of the TX signal is 0 degrees, the CMC 210 may operate in acapacitance sensing mode to taking a capacitive sensing measurement, asdiscussed below. When the CMC 210 operates in an inductance sensing modeto take an inductive sensing measurement, the phase of the TX signal maybe shifted 90 degrees, as discussed below.

The GPIO 226 may include a driver to adjust an amplitude of the TXsignal. For example, the GPIO 226 may include a first switch 231 and asecond switch 232. A switch may be a transistor, a gate, a device formaking or breaking the connection in a circuit, and so forth. In oneexample, when the GPIO 226 is coupled to a power source through a closedfirst switch 231 and an open second switch 232, the GPIO 226 may be setas high. In another example, when the GPIO 226 is coupled to groundthrough a closed second switch 232 and an open first switch 231, theGPIO 226 may be set as low. In another example, when the GPIO 226 isshorted to power and ground, the first switch 231 and the second switch232 are closed. In another example, when the switches 231 and 232 areboth open, the GPIO 226 may be set to a high impedance (high-Z) and mayoutput the signal received from the PMW 228. In one example, theprocessor 119 or the smart IO 230 may control the switches 231 and 232via the PWM 228.

In one embodiment, the GPIO 226 may be coupled to the sense capacitor222 (first and second electrodes). The charge measurement circuit 210can apply a TX signal on GPIO 226, which is coupled to the sense unit225. The TX signal may excite the sense capacitor 222. Exciting acircuit or component may refer to applying a voltage or current to thecircuit or component. For example, by applying a voltage of the TXsignal on a first electrode of the sense capacitor 222, a RX signal isinducted on a second electrode of the sense capacitor 222. The voltageor current of the RX signal may be measured by the charge measurementcircuit 210 via GPIO 220. As noted above, the RX signal may beintegrated by an analog to digital converter (ADC) as part of thedigitization of the RX signal. The processor 219 can further process thedigital signals as described herein.

To take a mutual capacitance measurement, the CMC 210 may measure amutual capacitance between a first electrode and a second electrode,where the mutual capacitance between the first electrode and the secondelectrode may be represented as sense capacitor 222. The device 200 mayalso include two capacitors for integration. In one embodiment, C_(Mod)214 may be used for mutual capacitance sensing. The current from C_(Mod)214 may be integrated onto C_(tank) 218 and the TX signal isdemodulated. In one example, C_(Mod) 214 may be used for selfcapacitance sensing and the current may be integrated onto C_(tank) 218.For mutual capacitance sensing, the C_(Mod) 214 and the C_(tank) 218 maybe used for capacitance sensing.

In another embodiment, the device 200 may include the resonant circuit224 coupled between the GPIO 226 and the sense capacitor 222. Theresonant circuit 224 and the sense capacitor 222 may form a sense unit225. The CMC 210 may apply the TX signal via GPIO 226 to the resonantcircuit 224 to excite the resonant circuit 224. FIGS. 3-6D illustratesdifferent embodiments of the resonant circuit 224, as discussed below.

The GPIO 220 may couple the TX signal sent from the GPIO 226 via theresonant circuit 224 and/or the capacitor 222 to the CMC 210. The signalreceived at the GPIO 220 may be referred to as a received (RX) signal.The CMC 210 may compare an amplitude of the RX signal to a referencesignal. For example, a converter of the CMC 210 may use the C_(Mod) 214as an integration capacitor. A charge may be added or removed from thecapacitor. For example, a current may be induced by voltage swings onthe capacitor from an electrode. The charge of the integration capacitormay be digitized at a converter of the CMC 210 to a first count. Thecharge measured at the capacitor 222 is also digitized by the converterinto a second count. The digitization of the measured charges to countsfunctions as a capacitance to digital converter.

The first count is compared to the second count to determine a relativedifference between the digital representations of the capacitance fromthe capacitors 214 and 218 and the capacitor 222. When a relativedifference between the first count and the second count exceeds athreshold, the difference in counts indicates a change in thecapacitance of the capacitor 222. The change in capacitance may indicatea presence of an object proximate to the capacitor 222. In one example,the difference in counts may be represented as the difference inamplitudes of the signals from the GPIO 220 and an integrated signalfrom the GPIOs 212 and 216.

In one example, when a relative difference between the counts does notexceed a threshold amount, the relative difference may indicate anobject is not proximate to the device 200. In another example, whenrelative difference between the counts does not exceed a thresholdamount, the device 200 may not be configured to sense the type of objectproximate to the device 200. In one example, when the device 200 isconfigured to perform capacitive sensing, the device 200 may not be ableto sense ferrous or non-ferrous metal objects. In another example, whenthe device 200 is configured to perform inductive sensing, the device200 may not be able to sense capacitive objects. In one embodiment, toperform the capacitive sensing, a phase of the TX signal may be 0degrees. In another embodiment, to perform the inductive sensing, aphase of the TX signal may be shifted. When the phase of the TX signalis shifted, the shift in phase changes how charge gets coupled to theresonant circuit 224 and the capacitor 222. In one example, the resonantcircuit 224 may not resonate when being used for capacitive sensing.When the resonant circuit 224 does not resonate, the inductor may not beexcited in the resonant circuit 224. In another example, the resonantcircuit 224, such as the inductor, may resonate when being used to forinductive sensing. The TX signal may be phase shifted provide that thecorresponding RX signal may be demodulated at the CMC 210. In oneexample, the phase of the TX signal may be shifted by approximately 80to 100 degrees. In another example, the phase of the TX signal may beshifted by at least 45 degrees. In another example, the phase of the TXsignal may be shifted by approximately 90 degrees. As the phase shiftdeparts farther from the 90-degree phase shift, a change in theamplitude of the RX signal at the GPIO 220 may be reduced. As the amountof charge is reduced by the departure from the 90-degree phase shift,the accuracy of the inductive sensing may be reduced.

The processor 219 may be coupled to the PWM 228. The processor 119 mayset the PWM 228 to set the phase of the TX signal generated by the CMC210 to a first phase for conductive sensing or a second phase forinductive sensing. The processor 119 may also set the CMC 210 to measurefor capacitive sensing or inductive sensing based on the phase of the TXsignal. In one example, the CMC 210 may measure current in the form ofcharge for the TX signal at the GPIO 220. The CMC may measure a chargeover time in the current. When the CMC 210 is set to measure the currentor voltage for capacitive sensing, the CMC 210 may determine a firstchange in the charge for a signal with a 0-degree phase shift indicatesa presence of an object. When the CMC 210 is set to a measure thecurrent for inductive sensing, the CMC 210 may determine a second changein the charge for a signal with a 90-degree phase shift indicates apresence of an object proximate the capacitor 222. In one embodiment,the CMC 210, the smart IO 230, the processor 119, the PWM 228, the GPIO226, the resonant circuit 224, the capacitor 222, the GPIO 533, the GPIO216, the GPIO 220, the C_(Mod) 214, and the C_(Tank) 218 may be locationon a single substrate, such as a single integrated circuit substrate ora common carrier substrate.

The CMC 210 may include a filter to filter the signal received at theGPIO 220. In one example, the filter may be an infinite impulse response(IIR) filter. In another example, the filter may be a decimator. In oneexample, the device 200 may be configured for capacitive sensing toreceive input from a user and may be configured for inductive sensing toreceive non-user inputs. For example, the device 200 may be configuredfor capacitive sensing when receiving user inputs via a touch screen andmay be configured for inductive sensing when receiving inputs from ametal button, an angular position sensor for control knobs, door opensensor, a drawer open/close sensor, a liquid level sensor, and so forth.

In one embodiment, for the automotive environment, inductive sensing maybe used to determine gear shifter positions and a capacitive sensor maybe used for touch panel force detection. The device 200 may beintegrated into a desktop computer, a laptop computer, a tabletcomputer, a personal digital assistant (PDA), a smartphone, a satellitenavigation device, a portable media player, a portable game console, akiosk computer, a point-of-sale device, a dishwasher, a washing machine,a liquid dispenser, a control panel on a household or other appliancethat includes a sense unit, and so forth.

FIG. 2B illustrates the sensing unit 225 that operates as a variablecapacitor in a capacitance mode when a frequency of the TX signal isbelow or above a resonant frequency, according to one embodiment. Whenthe frequency of the TX signal received at the resonant circuit 224differs from a resonant frequency of the resonant circuit 224 (such asbelow or above the resonance frequency), the resonant circuit 224 andthe sense capacitor 222 may output a current for capacitance sensing.For example, the TX signal excites a current of a coil of the resonantcircuit 224. The coil may interact with an induced current at theobject. The proximity of the object 237 may affect an inductance of thecoil. When the object 237 is grounded, a capacitance between the coiland the object may add to a total parasitic coupling of the coil to theground and the capacitance is distributed. When a TX frequency is not atthe resonant frequency, the resonant circuit may operate as a variablecapacitor in a capacitance mode that may relay an RX signal to the CMC210 for the CMC 210 to perform capacitance sensing.

FIG. 2C illustrates the sensing unit 225 that operates as a variableinductor in an inductance mode when the phase of the TX signal has beenshifted to excite a resonant circuit, according to one embodiment. Whenthe TX signal is at approximately a resonant frequency of the sensingunit 225 as shown in graph 235, the resonant circuit 224 and thecapacitor 222 may operate as an inductive sense unit in an inductancemode. When the object 237 is grounded, the capacitance between the coiland the object 237 may add to the parasitic coupling of the coil to theground and the capacitance is distributed. The electric current in thecoil interacts with an induced current at the metal object. Theproximity of the metallic object may affect the inductance of the coil.Additionally, when the frequency of the TX signal is at the resonantfrequency, the current may be maximized so that the resonant circuit isrelatively sensitive to a change in inductance as compared to a changein capacitance when operating in inductance mode.

FIG. 2D illustrates a circuit level diagram of the device 200 performingfull-wave capacitive sensing and inductive sensing, according to anembodiment. Some of the features in FIG. 2D are the same or similar tosome of the features in FIGS. 1 and 2A as noted by the same referencenumbers, unless expressly described otherwise. The device 200 mayperform full-wave sensing using the GPIOs 212, 216, 220, and 226, theresonant circuit 224, a CMC 216, a digital sequencer 233, a smart IO230, and/or a PWM 228.

The CMC 210 may generate a TX signal (also referred to as a csd_sensesignal). The device 200 may operate in a capacitive sensing mode or aninductive sensing mode. The digital sequencer 233 may control theswitches in the CMC 210 to configure the CMC 210 for capacitive sensingor inductive sensing. When the device 200 is operating in a capacitivesensing mode, the PWM 228 may not shift a phase of the TX signal that issent to the sensing unit 225 via the GPIO 226. When the device 200 isoperating in an inductive sensing mode, the PWM 228 may phase shift theTX signal before the TX signal is sent to the GPIO 226.

The GPIO 226 may be a driver that amplifies the TX signal or thephase-shifted TX signal when amplitudes of those signals is below athreshold amplitude level. The CMC 210 may send the TX signal via theGPIO 226 to the resonant circuit 224 at the node 251. The resonantcircuit 224 may be a sensor, such as an RLC circuit or an electrode,that the CMC 210 may use to take a capacitance measurement or aninductance measurement, as discussed above. The GPIO 220 may receive theRX signal from the capacitor 222 and couple the RX signal to the CMC210.

The CMC 210 may include an analog multiplexer (AMUX) 236, a balancer238, and a comparator 240. The AMUX 236 may combine or aggregate thesignals from the GPIO 212, the GPIO 216, and the GPIO 220 together andsend the combined signal to the balancer 238. The balancer 238 maybalance current sources of the CMC 210. For example, the balancer 238may be coupled to the digital sequencer 233 by a control line. Thebalancer 238 may receive control signals from terminals PHI1 252 andPHI2 254 of the digital sequencer 233 to balance the CMC 210.

The balancer 238 may also include a timing network (not shown). Thetiming network may couple positive and negative charges at the CMC 210with the correct phase. The balancer 238 may also include a demodulatorthat determines the counts of the CMC 210. The balancer 238 add chargeto the resonant circuit 224, the C_(Mod) 214, the C_(Tank) 218, and thecapacitor 222. In one example, the balancer 238 may add a charge back tothe C_(Mod) 214 and the C_(Tank) 218 to return the charge of the theC_(Mod) 214 and the C_(Tank) 218 to an initial charge level.

A converter may use the C_(Mod) 214 as an integration capacitor tostores or integrates charge from multiple transfer operations. Thecharge is digitized at the converter to a digital value of the current(RX) which is representative of the capacitance. The comparator 240 maycompare the digital value to a threshold to determine whether theamplitude of the RX signal has changed. The comparator 240 may compare adigital value representative of an amplitude of the RX signal from theGPIO 220 to a digital value representative of an amplitude of thereference signal at the comparator 240 to determine a relativedifference between the amplitudes of the voltages for the RX signal andthe reference signal. When the difference in the amplitudes exceeds athreshold amount, the difference may indicate a presence of an objectproximate to the capacitor 222.

The CMC 210 may combine measurements while the device 200 is set forcapacitive sensing and measurements are taken while the device 200 isset for inductive sensing. The CMC 210 may use the combined measurementinformation to distinguish between different types of objects. Forexample, the combined information can indicate whether an object is aplastic bottle or a metal can.

The digital sequencer 233 may control the switches in the CMC 210 toconfigure the CMC 210 for capacitive sensing or inductive sensing. Thesmart IO 230 may enable an internal interconnection of the digitalsequencer 233 with the PWM 228 or a TX signal driver. The digitalsequencer 233 may send a trigger input via the smart IO 230 to the PWM228 to trigger the PWM 228 to shift a phase of the TX signal. In anotherembodiment, the smart IO 230 may be an external terminal connector thatrelays the signal between the digital sequencer and the PWM 228.

The PWM 228 may shift a phase of the TX signal to excite the componentsof the resonant circuit 224 for capacitive sensing or inductive sensing.In one embodiment, the PWM 228 may maintain a 0-degree phase to excitethe resonant circuit 224 for capacitive sensing. In another embodiment,the PWM 228 may shift a phase of the TX signal by 90-degrees to excitethe resonant circuit 224 for inductive sensing.

In one example, the resistor R_(s) of the resonant circuit 224 may limitthe current flow. In another example, the TX signal may be a sine waveand the resistor R_(s) may set a peak-to-peak voltage of the sine wave.The capacitor 222 may couple the sine wave from the resonant circuit 224into the CMC 210. In one example, the resistor R_(s) may be a part of aGPIO and may be programmable. In another example, the GPIO could havedifferent drive strength control to achieve the resistance.

In one example, to reduce a power consumption of the resonant circuit224, a swing on the LC components of the resonant circuit 224 may not bemaximized and a relatively large resistor Rs may be used. The GPIO 220may receive the sine wave from the capacitor 222 and couple the RXsignal to a sensing channel of the CMC 210. The CMC 210 may convert theRX signal into a digital value.

In one embodiment, the CMC 210 may include a full-wave capacitivesensing converter. The 90-degree phase shift by the PWM 228 may enabledemodulation of the sine wave by the full-wave capacitive senseconverter.

FIG. 2E illustrates the CMC 210 of the device 200 that is configured forhalf-wave capacitive sensing and inductive sensing according to anembodiment. Some of the features in FIG. 2E are the same or similar tosome of the features in FIGS. 1 and 2A-2D as noted by the same referencenumbers, unless expressly described otherwise.

When the device 200 is in the half-wave capacitive sensing and inductivesensing configuration, the GPIO 216 and the C_(Tank) 218 may be removedfrom the device 200. The CMC 210 may include an analog multiplexer(AMUX) 242, a balancer 244, and a comparator 246. The AMUX 242 maycombine the signals from the GPIO 212 and the GPIO 220 together and sendthe combined signal to the balancer 244.

The balancer 244 may balance current sources of the CMC 210. Forexample, the balancer 244 may be coupled to the digital sequencer 233 bya control line. The balancer 244 may send control signals to the digitalsequencer 233 to balance the CMC. The balancer 244 may also include atiming network. The balancer 244 may also include a demodulator thatdetermines the digital values of the CMC 210 and accumulates a chargefor the resonant circuit 224, the C_(Mod) 214, and the capacitor 222.

The comparator 246 may compare a digital value for the RX signal fromthe GPIO 220 to a digital representation of an amplitude of thereference signal of the comparator 246 to determine a difference betweenamplitudes of the RX signal and the reference signal. When thedifference exceeds a threshold level, the difference may indicate apresence of an object proximate to the capacitor 222.

FIG. 3 illustrates a sense unit 300 where the resonant circuit 224includes a resistor 332, an inductor 334, a capacitor 336, a secondcapacitor 338, and a ground 340, according to an embodiment. Some of thefeatures in FIG. 3 are the same or similar to some of the features inFIGS. 1 and 2A-2E as noted by the same reference numbers, unlessexpressly described otherwise.

The resistor 332 may be coupled between the GPIO 226 and the node 342.The inductor 334, the capacitor 336, the second capacitor 338, and theground 340 may be components of a series circuit 344 that may be coupledto the node 342. In one example, the capacitor 336 and/or the secondcapacitor 338 may be discrete components. In another example, thecapacitor 336 and/or the second capacitor 338 may represent capacitancesformed between components. For example, the capacitor 336 may representa capacitance formed between inductor 334 and 340. The series circuit344 may be coupled between the resistor 332 and the capacitor 222. Thecapacitor 222 may be coupled between the node 342 and the CMC 210.

In one embodiment, the series circuit 344 may include the inductor 334that is in series with the capacitor 336 (i.e. L−C). The capacitor 336may be connected to the ground 340. In another embodiment, the seriescircuit 344 may include the inductor 334 that is in parallel with thesecond capacitor 338 (i.e. L∥C). The inductor 334 and the secondcapacitor 338 may be coupled to the ground 340.

The combination of the inductor 334 and the second capacitor 338 may bein series with the capacitor 336 (i.e. L∥C−C). The capacitor 336 may beconnected to the ground 340. The L∥C−C configuration may control theminimum and the maximum impedance of the resonant circuit 224. Forexample, the L∥C−C configuration may operate as an analog amplifier tocontrol the frequency range between the minimum and the maximumimpedance of the resonant circuit 224. The L∥C−C configuration mayprovide tuning for a resonant frequency of the sensing circuit.

The components of the series circuit 344 or the resonant circuit 224 arenot intended to be limiting. The resonant circuit 224 may include othercomponents or have other configurations.

FIG. 4A illustrates the sense unit 400 that includes a capacitor 412 andan inductor 416, according to an embodiment. The capacitor 412 and theinductor 416 may be coupled to a processing circuit 418. The Processingcircuit 418 may use the inductor 416 to detect the presence of an objectwhen operating in an inductive sensing mode.

In inductance sensing mode, the capacitor 412 may be a ground for theinductor 416. For example, in inductance sensing mode, a magnetic fieldmay be generated at the inductor 416 and when a signal is applied toinductor 416, the magnetic field induces a current at the inductor 416.When an object comes in proximity to the magnetic field, the object mayproduce an Eddy current that opposes the magnetic field.

The processing circuit 418 may use the capacitor 412 to detect thepresence of an object when operating in a capacitance sensing mode. Thefield lines 420 may illustrate the capacitance between the inductor 416and the capacitor 412. In a self-capacitive sensing mode, a capacitancemay be measured at the inductor 416. In a mutual capacitance sensingmode, capacitance may be measured between the inductor 416 and thecapacitor 412. For example, the inductor 416 may not be excited whenalternating current (AC) power is applied to the sense unit 400 and mayact as a grounded metal.

In one embodiment, the capacitor 412 may be a parallel plate that is ona first side of a substrate. The capacitor 412 may be connected to aground. The inductor 416 may be a spiral coil located on a second sideof the substrate. The inductor 416 may be located on a second side ofthe substrate. In the inductive mode, the capacitor 412 may be groundedand inactive. In the capacitive mode, the ground of the inductor 416couples the capacitive field to detect an object.

FIG. 4B illustrates the sense unit 401 of FIG. 4A that includes a flatcoil 422, according to an embodiment. Some of the features in FIG. 4Bare the same or similar to some of the features in FIG. 4A as noted bythe same reference numbers, unless expressly described otherwise. Theflat coil 422 may act as the inductor 416, such as a planar inductor.The flat coil 422 may have a relatively large inner diameter compared tothe outer diameter of the flat coil 422. The relatively large innerdiameter compared to the outer diameter of the flat coil 422 may providea relatively small capacitance coupling with an object proximate to theflat coil 422 because of the reduced surface area of the flat coil 422.The flat coil 422 may be coupled to a ground plate 424. In one example,the ground plate 424 may be located on a top side or a bottom side ofthe flat coil.

FIG. 4C illustrates the sense unit 402 of FIG. 4A that includes a flatcoil 426 with a small inner circumference, according to an embodiment.Some of the features in FIG. 4C are the same or similar to some of thefeatures in FIG. 4A as noted by the same reference numbers, unlessexpressly described otherwise. The flat coil 426 may act as the inductor416. The flat coil 426 may have a relatively small inner diametercompared to the outer diameter of the flat coil 426. The relativelysmall inner diameter compared to the outer diameter of the flat coil 426may provide a relatively high capacitance coupling with an objectproximate to the flat coil 426 because of an increased surface area ofthe flat coil 426.

FIG. 4D illustrates the sense unit 403 of FIG. 4A that includes arectangle coil 428, according to an embodiment. Some of the features inFIG. 4D are the same or similar to some of the features in FIG. 4A asnoted by the same reference numbers, unless expressly describedotherwise. The rectangle coil 428 may act as the inductor 416. Therectangle coil 428 may have a relatively small inner area compared tothe outer diameter of the rectangle coil 428. The relatively small innerarea compared to the outer area of the flat coil 426 may provide arelatively high capacitance coupling with an object proximate to therectangle coil 428 because of the increased surface area of therectangle coil 428.

FIG. 4E illustrates the sensing unit 404 of FIG. 4A that includes amultiple layer coil 430, according to an embodiment. Some of thefeatures in FIG. 4E are the same or similar to some of the features inFIGS. 2 and 4A as noted by the same reference numbers, unless expresslydescribed otherwise. The multiple layer coil 430 may include a firstcoil 432 and a second coil 434. The first coil 432 may be located abovethe second coil 434 along a Z axis. In one example, when the sense unit400 is used for capacitance sensing, the coil 432 may be used forcoupling with the capacitor 412. In another example, when the device 200is used for inductance sensing, the coil 432 and the coil 434 may beused for inductive coupling.

FIG. 4F illustrates the sense unit 405 of FIG. 4A that includes aprimary coil 438 and a secondary coil 440, according to an embodiment.Some of the features in FIG. 4F are the same or similar to some of thefeatures in FIG. 4A as noted by the same reference numbers, unlessexpressly described otherwise. An outer circumference of the secondarycoil 440 may be less than an inner circumference of the primary coil438. The secondary coil 440 may be coplanar and located within the innercircumference of the primary coil 438.

FIG. 4G illustrates the sense unit 406 of FIG. 4A that includes theprimary coil 438 and the secondary coil 440 that are coplanar, accordingto an embodiment. Some of the features in FIG. 4G are the same orsimilar to some of the features in FIGS. 2, 4A, and 4H as noted by thesame reference numbers, unless expressly described otherwise. An outercircumference of the secondary coil 440 may be approximately the same asthe outer circumference of the primary coil 438. The secondary coil 440may be located coplanar and adjacent to the primary coil 438. Theimplementations in FIGS. 4A-4G are not intended to be limiting. Forexample, the sense unit 400 may include a spiral coil, a solenoid coil,a triangle coil, a stretched coil, and so forth.

FIG. 5 illustrates a sensing circuit 500 with a hybrid capacitive andinductive sensor, according to an embodiment. Some of the features inFIG. 5 are the same or similar to some of the features in FIGS. 1 and2A-2E as noted by the same reference numbers, unless expressly describedotherwise.

The sensing circuit 500 may include the PWM 228, the GPIO 226, and asense unit 540. The sense unit 540 may include at least one of aresistor 511, a first LC circuit 512, a GPIO 514, a second LC circuit516, a capacitor 518, a ground 520, a capacitor 522, the GPIO 533, theGPIO 220, or the CMC 210. The first LC circuit 512, the second GPIO 514,the second LC circuit 516, the GPIO 533, and the GPIO 220 may beconfigurable to provide various configurations of the sensing circuit500 for capacitive sensing and inductive sensing.

The CMC 210 may include a signal generator. The signal generator maygenerate a TX signal. In one example, the TX signal may be a square wavesignal. An initial phase of the TX signal generated by the signalgenerator may be relative to a reference signal at the CMC 210. Thephase shifter of the PWM 228 may control a phase of the TX signal. TheCMC 210 may use the sense unit 540 for capacitive sensing when there isnot a phase shift. The CMC 210 may use the sense unit 540 for inductivesensing when there is a phase shift.

The GPIO 226 may be a driver to adjust an amplitude of the TX signal.The GPIO 226 may be coupled to the resistor 511. The TX signal may besent to the CMC 210 as an RX signal via the resistor 511, a firstcircuitry 528, a second circuitry 530, the capacitor 522, the GPIO 220,and/or the GPIO 533. The TX signal may excite the resistor 511, a firstcircuitry 528, a second circuitry 530, the capacitor 522, the GPIO 220,and/or the GPIO 533.

In one embodiment, the resistor 511 may be coupled to a node 524. Inanother embodiment, a first circuitry 528 may be coupled to the node524. The LC circuit 512 and the GPIO 514 may be part of the firstcircuitry 528. The GPIO 514 may include a ground or may be coupled to aground. The LC circuit 512 and the GPIO 514 may be connected in aseries. The first circuitry 528 may be series to the resistor 511.

In another embodiment, the resistor 511 or the first circuitry 528 maybe coupled in parallel to a node 526. For example, the second LC circuit516 and the capacitor 518 may be part of a second circuitry 530 that iscoupled to the node 526. The capacitor 518 may also be coupled to aground. The second LC circuit 516 and the capacitor 518 may be connectedin series. In one embodiment, the second circuitry 528 may be in serieswith the resistor 511. In another embodiment, the second circuitry 528may be parallel to the first circuitry 528.

The resistor 511, the first circuitry 528, and/or the second circuitry530 may be coupled to a node 532. The capacitor 522 may be coupled tothe node 532 in series with the resistor 511, the first circuitry 528,and/or the second circuitry 528. The GPIO 220 may be coupled in serieswith the capacitor 522 and the CMC 210. In one embodiment, the GPIO 533may be coupled to the node 532 and may be in parallel with the capacitor522 and the GPIO 220. The GPIO 533 may also be coupled to the CMC.

When the TX signal is sent across the resistor 511, the first circuitry528 and/or the second circuitry 528, and the capacitor 522, theresulting signal may be referred to as an RX signal. The GPIO 220 and/orthe GPIO 533 may couple the RX signal received from the GPIO 226 via theresistor 511, the first circuitry 528, and/or the second circuitry 528into the CMC 210. The CMC 210 may compare the digital valuecorresponding to the RX signal with a digital value of a referencesignal to determine whether there is a relative difference between anamplitude of the RX signal and an amplitude of the reference signal thatexceed a threshold value. When the relative difference between therepresentative amplitudes of the RX signal and the reference signalexceeds the threshold amount, the difference may indicate a presence ofan object proximate to the device 200.

In one example, when a difference between the RX signal and thereference signal does not exceed the threshold amount, the differencemay indicate an object is not proximate to the sensing circuit 500, suchas the LC circuit 512 or the LC circuit 516. In another example, when adifference between the RX signal and the reference signal does notexceed a threshold amount, the sensing circuit 500 may not be configuredto sense the type of object proximate to the device 200 and may notindicate that an object is proximate to the sensing circuit 500. In oneexample, when the sensing circuit 500 is configured to performcapacitive sensing, the sensing circuit 500 may be able to sense ferrousor non-ferrous conductive objects. In another example, when the sensingcircuit 500 is configured to perform inductive sensing, the sensingcircuit 500 may be able to sense conductive objects at a potential. Thesensing circuit 500 may use the first circuitry 528 and/or 530 toperform the inductive sensing and the capacitor 522 to perform thecapacitive sensing. The components of the first circuitry 528 or 530 arenot intended to be limiting. FIGS. 6A-6D illustrates differentimplementations of the resonant circuits 528 and 530 and the GPIOs 533and 220, as discussed below.

FIG. 6A illustrates sensing circuit 600 including the sense unit 540 ofFIG. 5 that includes the first circuitry 528 with a capacitor 640 andthe second circuitry 530 with an inductor 642, according to anembodiment. In one embodiment, sensing circuit may be referred to as atwo sensor hybrid sensing circuit, which may be configured respectivelyfor inductive sensing and capacitive sensing. Some of the features inFIG. 6A are the same or similar to some of the features in FIG. 5 asnoted by the same reference numbers, unless expressly describedotherwise. The resistor 511 and the first circuitry 528 may be connectedin series and may be connected to the node 524. The node 524 may beconnected to node 526. The second circuitry 530 may be connected to thenode 526. The second circuitry 530 may also be connected in parallelwith the first circuitry 528. The inductor 642 of the first circuitry530 may be a first electrode. The first circuitry 528 and the secondcircuitry 530 may be connected to a common alternating current (AC)ground.

The capacitor 518 may be connected between the node 526 and the GPIO220. The GPIO 220 may be connected to the CMC 210. The CMC 210 may beconnected to the resonant circuit 646 by a switch 645. When the switch645 is open, the resonant circuit 646 is disconnected from the CMC 210.When the switch 645 is closed, the resonant circuit 646 may be coupledto the CMC 210. In one embodiment, the resonant circuit 646 is a secondelectrode.

When sensing circuit 600 is configured for inductive sensing, the firstcircuitry 528 and the second circuitry 530 may be connected to the CMC210 via the capacitor 518. The CMC 210 and the GPIO 226 may beconfigured for inductive sensing mode. In one example, a metal objectproximate to the sensing circuit 600, such as capacitor 640 in parallelwith the inductor 642 may have an impact on the amplitude and phase ofthe LC circuitry including first circuitry 528 and inductor 642, whichcan be detected by the CMC 210. The CMC 210 may detect the relativechange in the amplitude of the RX signal which may be detected by theCMC 210 in the inductive sensing configuration.

In one embodiment, when the sensing circuit 600 is configured forinductive sensing, the resonant circuit 646 may be disconnected from theCMC 210 by switch 645. For example, the resonant circuit 646 may be anelectrode and the switch 645 may set the electrode connection either toground or to high-Z. The CMC 210 and the GPIO 226 may be configured forinductive sensing. The CMC 210 may use the RX signal that is received atthe GPIO 220 to detect ferrous or non-ferrous metal objects that changean amplitude of the RX signal.

In another embodiment, the second circuitry 530 may be a flat circularshaped electrode. In another embodiment, the resonant circuit 646 may bea spiral shaped electrode. The shape of the electrodes is not intendedto be limiting. For example, the electrodes may be shaped to have athreshold level of capacitive coupling between the inductor 642 and theresonant circuit 646 for capacitive sensing.

FIG. 6B illustrates an embodiment of an one sensor hybrid sensingcircuit of the sense unit 540 of FIG. 5 that includes the firstcircuitry 528 with the capacitor 640 and the second circuitry 530 withthe inductor 642, a capacitor 648, and a ground 650, according to anembodiment. In alternative embodiments, the one sensor hybrid sensingcircuit may be configured for capacitive sensing (in FIG. 6B) andinductive sensing (in FIG. 6C). Some of the features in FIG. 6B are thesame or similar to some of the features in FIGS. 5 and 6A as noted bythe same reference numbers, unless expressly described otherwise.

The resistor 511 and the first circuitry 528 may be connected in seriesat the node 524. The node 524 may be connected to node 526. The secondcircuitry 530 may be connected to the node 526. The second circuitry 530may include the inductor 642, the capacitor 648, and the ground 650connected in a series.

The second circuitry 530 may also be connected in parallel with thefirst circuitry 528. In one embodiment, second circuitry 530 may beconnected in series with the first circuitry 528 (not shown in FIG. 6B).The capacitor 518 may be connected between the node 526 and the GPIO220. In one example, the GPIO 220 may be a switch with an open positionand a closed position. When the switch is closed the capacitor 518 isconnected to the CMC 210. When the switch is opened the capacitor 518 isdisconnected from the CMC 210. The GPIO 651 may also be a switch. Whenthe GPIO 651 is closed, the CMC 210 may be connected directly to node526. When the GPIO is open, the CMC 210 may be disconnected from thenode 526. In one embodiment, the first circuitry 528 and the secondcircuitry 530 may be connected to the CMC 210 via coupling capacitor 518and the GPIO 651 may be open.

When the sensing unit 540 is configured for capacitance sensing, thecapacitor 518 may be bypassed with GPIO 220. The switch 656 disconnectsthe inductive sensing circuitry from receiving the TX signal and setsthe inactive GPIO 226 connection to a High-Z. The switch 658 disconnectsa ground from the sensing circuit 500 and sets the inactive groundconnection to high-Z. In this configuration, the CMC 210 may beconfigured for self-capacitive sensing (CSD), where the inductor 642 maybe configured as a self-capacitance electrode. Object approximate to theinductor 642 may alter the self-capacitance of inductor 642 which may bedetected by the CMC 210 operating in the CSD configuration.

FIG. 6C illustrates the sense unit 540 where the GPIO 651 is open andthe GPIO 220 is closed, according to an embodiment. The embodiment inFIG. 6C may be similar to the one sensor hybrid sensing circuit in FIG.6B but configured for inductive sensing. Some of the features in FIG. 6Care the same or similar to some of the features in FIGS. 5, 6A, and 6Bas noted by the same reference numbers, unless expressly describedotherwise. The resistor 511 and the first circuitry 528 may be connectedin series by the node 524. The first circuitry 528 may be the capacitor640. The node 524 may be connected to node 526. The second circuitry 530may be connected to the node 526. The second circuitry 530 may includethe inductor 642. The first circuitry 528 and the second circuitry 530may be connected in parallel and may be connected to an AC ground. In analternative embodiment, the first circuitry 528 and the second circuitry530 may be connected in series.

The capacitor 518 may be connected to the node 526 and the GPIO 220. Inone example, the GPIO 220 may be a switch with an open position and aclosed position. When the switch is in the closed position the capacitor518 is connected to the CMC 210. When the switch is in the open positionthe capacitor 518 is disconnected from the CMC 210. The GPIO 651 mayalso be a switch. When the GPIO 651 is closed the CMC 210 may bedirectly connected to node 526. When the GPIO is open the CMC 210 may bedisconnected from the node 526 or may be connected to the node 526 viathe GPIO 220.

In one embodiment, when the sense unit 540 is configured for inductivesensing, the inductor 642 may be connected to the CMC 210 by GPIO 220.GPIO 651 may be open. The CMC 210 and the GPIO 226 may be configured forinductive sensing. The CMC 210 may use the RX signal that is received atthe GPIO 220 to detect ferrous or non-ferrous metal objects that changean amplitude of the RX signal.

FIG. 6D illustrates the sense unit 540 of FIG. 5 that includes a firstcircuitry 528 with a capacitor 514 and the second circuitry 530 with theinductor 642, a capacitor 662, an inductor 664, a capacitor 666, and aground 668, according to an embodiment. The embodiment in FIG. 6D may besimilar to the two sensor hybrid sensing circuit shown in FIG. 6A butconfigured for capacitive sensing. Some of the features in FIG. 6D arethe same or similar to some of the features in FIGS. 1, 2, 5, 6A, 6B,and 6C as noted by the same reference numbers, unless expresslydescribed otherwise.

The GPIO 226 may be connected to a switch 660. When the switch 660 is inthe closed position, the GPIO 226 is connected to the resistor 511. Whenthe switch 660 is open the GPIO 226 is disconnected from the GPIO 226.The resistor 511 and the first circuitry 528 may be connected in seriesby the node 524. The first circuitry 528 may be the GPIO 514.

The node 524 may be connected to the node 526. The second circuitry 530may be connected to the node 526. The second circuitry 530 may also beconnected in parallel with the first circuitry 528. The inductor 642 andcapacitor 640 may be connected to an AC ground. The second circuitry 530may include the inductor 642, the capacitor 662, the inductor 664, thecapacitor 666, and the ground 668. The inductor 642 may be coupled inseries with the capacitor 662. The capacitor 662 may be coupled inseries with the resonant circuit 646. The inductor 664 may be coupled inseries with the capacitor 666. The capacitor 666 may be coupled inseries with the ground 668.

The capacitor 518 may be connected between the node 526 and the GPIO220. The GPIO 220 may be connected to the CMC 210. When the GPIO 220 isin an open position, the capacitor 518 may be disconnected from the CMC210. When the GPIO 220 is in a closed position the capacitor may beconnected to the CMC 210.

In one embodiment, when the sense unit 540 is configured for capacitancesensing, the resonant circuit 642 may be disconnected from the CMC 210by GPIO 220 and third resonant circuit 642 may be connected to the CMC210 via the capacitor 662 and the resonant circuit 646. For example, theGPIO 220 may set the third resonant circuit 642 to ground or to High-Z.The switch 660 may disconnect the GPIO 226 so that the sense unit 540does not receive the TX signal. The switch 660 may set the connectionfor the TX signal to ground or to high-Z.

In one embodiment, when the CMC 210 is configured for self-capacitivesensing, the third resonant circuit 642 may be grounded and beconfigured as a second electrode to couple with the resonant circuit646. The CMC 210 may use the RX signal that is received to detectconductive objects that may change a capacitive field between the thirdresonant circuit and the resonant circuit 646.

FIG. 7 illustrates a graph 700 of an amplitude change 720 associatedwith a digital representation of the RX signal 712, according to anembodiment. The graph 700 shows a digital representation of the RXsignal 712 that is received at the CMC 210 in FIG. 2A. The resonantcircuit 224 and the capacitor 222 of FIG. 2A may receive a TX signal attime period 714 and may be excited by the TX signal.

At time period 714, line 710 shows no relative change between theamplitudes the RX signal and the reference signal. No relative changemay indicate that no object may be detected by the CMC 210 whenperforming capacitive sensing for the non-phased shifted signal, asdiscussed above. At time period 716, the PWM 228 may shift the phase ofthe TX signal by approximately 90 degrees. At time period 716, line 710shows a relative change between the amplitudes of the RX signal and thereference signal. The relative change may indicate that an object may bedetected by the CMC 210 when performing inductive sensing using thephase-shifted signal, as discussed above. In one embodiment, the peaks722 of the RX signal 712 may show a period of time where an object isbeing placed proximate to the device 200.

At time period 718, the PWM 228 may shift the phase of the TX signalback to approximately 0 degrees. At time period 718, line 710 shows norelative change in an amplitude between the RX signal and the referencesignal. No relative change may indicate that object may not be detectedby the CMC 210 when performing capacitive sensing using the non-phaseshifted signal, as discussed above.

In one embodiment, the PWM 228 may alternate between sending a TX signalwith a 0-degree phase and a phase-shifted TX signal. A timing of the CMC210 may be synchronized to alternate a phase of the TX signal so thatthe CMC 210 may be set to perform capacitive sensing or inductivesensing sequentially.

In another embodiment, the device 200 may switch between capacitivesensing and inductive sensing based on an application the device 200 isbeing used for. For example, a device with the device 200 may have a lowpower mode that uses capacitive sensing for a power button. Once thedevice is turned on, the device may switch to using inductive sensing toreceive user input.

In another embodiment, the device 200 may take capacitive sensingmeasurements until a signal to noise (SNR) level of the RX signal, whichis an input charge (Vtank*Cc), exceeds a threshold SNR level. When theSNR level exceeds the threshold SNR level, the processor 119 in FIG. 2Amay switch the PWM 228 and the CMC 210 to inductive sensing. When thedevice 200 is performing inductive sensing and the SNR level of the TXsignal exceeds the threshold SNR level, the processor 119 may similarlyswitch the PWM 228 and the CMC 210 to capacitive sensing.

In one embodiment, the threshold SNR level may vary based on a size ortype of object the device 200 it configured to sense. For example, whena size of the object is relatively small, the threshold SNR level may beincreased to provide the device 200 sufficient time to performcapacitive or inductive sensing. More time may be needed because the TXsignal may be noiser and it may take the CMC 210 longer to averageseveral capacitive or inductive measurements to determine a presence ofthe object. Measuring for relatively smaller objects may take a longeramount of time than relatively large objects because the amount ofcoupling between the relatively small objects and the device 200 issmaller.

FIG. 8A illustrates a graph 800 of a phase shifting and demodulation ofa TX signal 810 for inductive sensing, according to an embodiment. Asdiscussed above, the resonant circuit 224 in FIG. 2B may receive the TXsignal 810 from the node 250 of the GPIO 226 which excites thecomponents of the resonant circuit 224. For inductive sensing, the PWM228 of the FIG. 2A phase shifts the TX signal.

In graph 800 for inductive sensing, the TX signal 810 may be phaseshifted by approximately 90 degrees. The Vamp signal 812 may show achange in voltage at an inductor of the resonant circuit 224 and thecapacitor 222. The Vamp signal 812 may be received at the GPIO 220. TheVamp signal 812 may be a sine wave as the inductor may not respondinstantaneously to a change between a high voltage and a low voltage ofthe TX signal 810. In one example, the PWM 228 may phase shift the TXsignal by 90 degrees with respect to the demodulation clocks signalsPHI1 814 and PHI2 816. The demodulation clock signals PHI1 814 and PHI2816 may be internal signals that feed from PHI1 252 and PHI2 254 of thedigital sequencer 233 into the demodulator of the CMC 210 in FIG. 2A. Inone embodiment, there may be a dead band between the PHI1 814 and PHI2816. The dead band may be an interval of a signal domain or band whereno action occurs. In another embodiment, the PHI1 814 and the PHI2 816may control the switches of the CMC 210 in FIGS. 2A-6D.

The phase shifted TX signal may be phase shifted so that the peak of thesine wave Vamp signal 818 occurs 90° after a rising edge of the initialTX signal. The phase-shifted TX signal may be fully integrated by theCMC 210. The demodulator of the CMC 210 may switch the signals 820received by the CMC 210 from the GPIOs 212, 216, and/or 220 to generatean aggregated signal 822 with the same phase. For example, the CMC 210may add together the positive and negative portions of the signals fromthe GPIOs 212, 216, and 220 to obtain the signal 822. In this example,as the CMC 210 pushes and pulls charge from the resonant circuit 224,the CMC 210 may aggregate and integrate the signals together over timeto generate a V_(integrate) signal 824. The aggregation and integrationof the signals may increase the amount of charge applied to the resonantcircuit 224. The CMC 210 may also convert the signal 822 to a digitalsignal using an analog to digital converter (ADC).

The V_(integrate) signal 824 may be a virtual voltage that illustratesan accumulation of a signal voltage. In one example, digital values fromthe converter may be accumulated using a counter, and the digital valuesmay be sent from terminal 256 of the comparator 240. Each cycle isintegrated and converted and the integration capacitors are returned toa value (V_(ref)) that the integration capacitors started at. The CMC210 may use the digital values to apply a signal at the resonant circuit224 to excite the components of the resonant circuit 224 for inductivesensing.

FIG. 8B illustrates a graph 802 with a TX signal 810 for capacitivesensing, according to an embodiment. The TX signal 810 may not be phaseshifted with respect to the demodulation clocks signals PHI1 814 andPHI2 816. The demodulation clock signals PHI1 814 and PHI2 816 may beinternal signals that feed from PHI1 252 and PHI2 254 of the digitalsequencer 233 into the demodulator of the CMC 210 in FIG. 2A. TheV_(ref) 826 is the voltage seen from the signal of the capacitor 218 atpoint 255 in FIG. 2D. The V_(ref) 828 is the voltage seen from thesignal of the capacitor 214 at point 257 in FIG. 2D. The V_(ref) 830 isthe voltage seen from the signal of the GPIO 220 at point 259 in FIG.2D. A count duration 832 is representative of the V_(ref) 826, theV_(ref) 828, and the V_(ref) 830 may indicate whether an object isproximate to a sense unit.

In one example, digital values from the converter may be accumulatedusing a counter, and the digital values may be sent from terminal 256 ofthe comparator 240. Each cycle is integrated and converted into adigital value using the V_(DD) at point 261 of FIG. 2D. The CMC 210 mayuse the digital values to determine whether an object is proximate thesense unit. For example, the CMC 210 may determine that an object isproximate the sense unit when a counter duration of the digital valueschanges.

FIG. 9 illustrates a graph 900 showing the phase shifting anddemodulation of a resonant circuit output signal for inductive sensing,according to an embodiment. Some of the features in FIG. 9 are the sameor similar to some of the features in FIG. 8 as noted by the samereference numbers, unless expressly described otherwise. The signal 912shows that the PHI1 814 and the PHI2 816 may be combined into ademodulation clock signal 912 that feeds into the demodulator of the CMC210 in FIG. 2A. The signal 912 may control the switches of the CMC 210in FIGS. 2A-6D.

In one embodiment, during the PHI2 clock phase of the demodulation clocksignal 912, a charge on an AMUX of the CMC 210 may rise above below areference voltage (V_(ref) HI). During the PHI1 clock phase the digitalsequencer 233 in FIG. 2A may discharge the C_(Tank) 218 back below theV_(ref) HI. During the high phase of the demodulation clock signal 912,the voltage on the AMUX may increase above the V_(ref) HI voltage.During the PHI2 clock phase, the digital sequencer 233 charges theC_(Mod) 214 of FIG. 2A down to the reference voltage.

FIG. 10A shows a graph 1000 of the frequencies used by the CMC 210 inFIG. 2A for inductive sensing, according to an embodiment. An electrodemay be used for inductive sensing. The sensing range of an electrode maybe dependent on the type of object being detected and the size and shapeof the electrode. For example, ferrous metals, such as iron and steel,may allow for a longer sensing range, while nonferrous metals, such asaluminum and copper, can reduce a sensing range of the electrode by upto 60 percent.

In one embodiment, selecting a size and shape of an electrode mayinclude determining an proximate to D_inner/D_outer ratio of theelectrode, determining an inductance per turn (AL) for the electrode asthe functions of the D_inner, D_Outer, thickness, trace and space width,number of turns and layers, and layout of the layers of the electrode.The size and shape of an electrode may vary based on the application itis used in. For example, the electrode shape may be a flat coil or apancake spiral coil. When the electrode is used for capacitive sensing,a high potential may be applied to an outer terminal of the electroderather than to the central or inner pad. A ratio of an inner diameter(D_inner) of the electrode to an outer diameter (D_outer) of theelectrode may be based on the size of the object being detected and theoperating distance between the electrode and object. In one example, theD_inner/D_outer ratio may be approximately 0.25 for objects relativelyclose to the electrode. In another example, the D_inner/D_outer ratiomay be approximately 0.6 for objects relatively far from the electrode.

In another embodiment, selecting the size of the electrode may alsoinclude determining a coil capacitive coupling between the turns andlayers of the electrode and between the ground and a target object. Theinductance and resistance of the electrode may then be analyzed toselect a size of the electrode with an optimal frequency response.

The optimal frequency of operation is where a signal difference betweenan object and no object is largest and where the digital values changeby the largest amount. In one example, a resonant circuit may be excitedby a TX signal at a resonant frequency of the resonant circuit. In oneexample, the resonant frequency of the resonant circuit may be ½*π*LC.In another example, the farther the frequency moves away from theresonant frequency, the larger a difference in the signals may be. Inanother example, a series resistance in the resonant circuit 224 in FIG.2A may be changed to amplify or attenuate the amount the difference inthe signals.

The graph 1000 shows an amplitude of a reference signal 1012 and anamplitude of the RX signal 1010. At points 1014 and 1016, the differencein amplitude between the reference signal 1012 and the RX signal 1010 isthe greatest indicating a greatest difference in the digital values. Thedifferences in amplitudes indicate that approximately 600 kilohertz(kHz) or 1000 kHz is the optimal frequency.

FIG. 10B shows a graph 1020 another frequency that may be used by theCMC 210 in FIG. 2A for inductive sensing, according to an embodiment.The graph 1020 shows an amplitude of a reference signal 1024 and anamplitude of the TX signal 1022. At point 1026, the difference inamplitude between the reference signal 1024 and the TX signal 1022 isthe greatest. The differences in amplitudes indicate that approximately1000 kHz is the optimal frequency.

FIG. 10C shows a graph 1030 another frequency used by the CMC 210 inFIG. 2A for inductive sensing, according to an embodiment. The graph1030 shows an amplitude of a reference signal 1034 and an amplitude ofthe TX signal 1032. At point 1036, the difference in amplitude betweenthe reference signal 1034 and the TX signal 1032 is the greatest. Thedifferences in amplitudes indicate that approximately 1000 kHz is theoptimal frequency.

FIG. 11 illustrates a flow diagram of a method 1100 of determining aninductance or a capacitance of a sensing unit, according to oneembodiment. The method 1100 may be performed by processing logic thatcomprises hardware (e.g., circuitry, dedicated logic, programmablelogic, microcode, etc.), software (such as instructions run on aprocessing device), or a combination thereof. The method 1100 may beperformed all or in part by device 200.

Method 1100 begins at block 1110 where a signal generator generates afirst signal and a third signal. Method 1100 continues at block 1120where the PWM may shift a phase of the first signal to obtain the thirdsignal. In one example, the PWM may shift the phase of the third signalby approximately 90 degrees relative to a phase of a reference signal.Method 1100 continues at block 1130 where the first signal excites asense unit. Method 1100 continues at block 1140 where a charge measuringcircuit may measure a second signal on a sense unit when the firstsignal is applied to the sense unit by the signal generator. The secondsignal may be representative of a capacitance of the sense unit. Method1100 continues at block 1150 the third signal may excite the sense unit.Method 1100 continues at block 1160 where the charge measuring circuitmay measure a fourth signal on a sense unit when the third signal isapplied to the sense unit by the signal generator. The fourth signal maybe representative of an inductance of the sense unit.

FIG. 12 illustrates a flow diagram of a method 1200 of applying signalsto a first electrode and a second electrode, according to anotherembodiment. The method 1200 may be performed by processing logic thatcomprises hardware (e.g., circuitry, dedicated logic, programmablelogic, microcode, etc.), software (such as instructions run on aprocessing device), or a combination thereof. The method 1200 may beperformed all or in part by device 200.

Method 1200 begins at block 1210 where a first signal may be applied toa first electrode in a first mode. Method 1200 continues at block 1220where a second signal is received at a second electrode in response tothe first signal being applied at the first electrode. The second signalmay be indicative of a capacitance between the first electrode and thesecond electrode. Method 1200 continues at block 1230 where a thirdsignal is applied to an inductive coil in a second mode. Method 1200continues at block 1240 where a fourth signal is received at theinductive coil in response to the third signal. The fourth signal isindicative of an inductance of the inductive coil. The fourth signal maybe indicative of an inductance of the second electrode. Method 1200continues at block 1240 where a CMC may determine that an object isproximate to the first electrode, the second electrode, or the inductivecoil. In one example, the CMC may determine that a capacitive object isproximate to the first electrode or the second electrode when the secondsignal changes. The change in the second signal is indicative of achange in the capacitance between the first electrode and the secondelectrode. In another example, the CMC may determine that a ferrousmetal object or a non-ferrous metal object is proximate to the inductivecoil when the fourth signal changes. The change in the fourth signal mayindicative of a change in inductance at the inductive coil.

Embodiments of the present invention include various operationsdescribed herein. These operations may be performed by hardwarecomponents, software, firmware, or a combination thereof.

Although the operations of the methods herein are shown and described ina particular order, the order of the operations of each method may bealtered so that certain operations may be performed in an inverse orderor so that certain operation may be performed, at least in part,concurrently with other operations. In another embodiment, instructionsor sub-operations of distinct operations may be in an intermittentand/or alternating manner. The terms “first,” “second,” “third,”“fourth,” etc. as used herein are meant as labels to distinguish amongdifferent elements and may not necessarily have an ordinal meaningaccording to their numerical designation.

The above description sets forth numerous specific details such asexamples of specific systems, components, methods, and so forth, inorder to provide an understanding of several embodiments of the presentinvention. It may be apparent to one skilled in the art, however, thatat least some embodiments of the present invention may be practicedwithout these specific details. In other instances, well-knowncomponents or methods are not described in detail or are presented insimple block diagram format in order to avoid unnecessarily obscuringthe present embodiments. Thus, the specific details set forth are merelyexemplary. Particular embodiments may vary from these exemplary detailsand still be contemplated to be within the scope of the presentinvention.

What is claimed is:
 1. A sensing circuit comprising: a first terminal tocouple to a first electrode via a resonant circuit; a second terminal tocouple to a second electrode; a signal generator coupled to the firstterminal, the signal generator to generate a first transmit signal in afirst mode and a second transmit signal in a second mode; and a receiverchannel coupled to the second terminal, the receiver channel to receivea first receive signal in the first mode and a second receive signal inthe second mode, wherein: the first transmit signal has a firstfrequency that is less than or greater than a resonant frequency of theresonant circuit; the second transmit signal has a second frequency thatis equal to the resonant frequency; the first receive signal isindicative of a variable capacitance in the first mode; and the secondreceive signal is indicative of a variable inductance in the secondmode.
 2. The sensing circuit of claim 1, wherein the variablecapacitance is a mutual capacitance formed between the first and secondelectrodes.
 3. The sensing circuit of claim 1, wherein the resonantcircuit comprises an inductive coil, wherein the first electrode iscoupled to a first side of the inductive coil, and a second side of theinductive coil is coupled to the first terminal.
 4. The sensing circuitof claim 3, wherein a magnetic field is generated on the inductive coilin response to the second transmit signal in the second mode.
 5. Thesensing circuit of claim 4, wherein an object proximate to the inductivecoil induces an Eddy current that opposes the magnetic field, whereinthe Eddy current changes the variable inductance, represented by thesecond receive signal.
 6. The sensing circuit of claim 3, wherein theresonant circuit further comprises a capacitor coupled in series withthe inductive coil.
 7. The sensing circuit of claim 1, wherein thesignal generator is a pulse width modulator (PWM).
 8. The sensingcircuit of claim 1, wherein the receiver channel comprises a chargemeasurement circuit coupled to the second terminal, wherein the chargemeasurement circuit is to: output a first value indicative of thevariable capacitance between the first and second electrodes in thefirst mode; and output a second value indicative of the variableinductance of the first and second electrodes in the second mode.
 9. Thesensing circuit of claim 1, wherein the receiver channel comprises acharge measurement circuit coupled to the second terminal, wherein thecharge measurement circuit has a first configuration in the first modeand a second configuration in the second mode.
 10. The sensing circuitof claim 1, wherein the second transmit signal is shifted approximately90 degrees relative to a phase of the first transmit signal.
 11. Amethod of operating a sensing circuit, the method comprising:outputting, on a first terminal, a first transmit signal to a sensingunit in a first mode, the sensing unit comprising a resonant circuit, afirst electrode, and a second electrode, wherein the first transmitsignal has a first frequency that is less than or greater than aresonant frequency of the resonant circuit; outputting, on the firstterminal, a second transmit signal to the sensing unit in a second mode,wherein the second transmit signal has a second frequency that is equalto the resonant frequency; receiving, on a second terminal, a firstreceive signal from the sensing unit in the first mode, wherein thefirst receive signal is indicative of a capacitance of the sensing unit;and receiving, on the second terminal, a second receive signal from thesensing unit in the second mode, wherein the second receive signal isindicative of an inductance of the sensing unit.
 12. The method of claim11, further comprising: determining that a conductive object isproximate to the first electrode or the second electrode when the firstreceive signal changes, wherein a change in the first receive signal isindicative of a change in the capacitance of the sensing unit; ordetermining that a ferrous or non-ferrous metal object is proximate tothe sensing unit when the second receive signal changes, wherein achange in the second receive signal is indicative of a change in theinductance of the sensing unit.
 13. The method of claim 11, wherein thesecond transmit signal is an alternating current (AC) signal.
 14. Themethod of claim 11, wherein the second transmit signal is phase shiftedby 90 degrees relative to the first transmit signal.
 15. The method ofclaim 11, wherein the first transmit signal causes the sensing unit tooperate as a variable capacitor in the first mode when the firstfrequency is less than or greater than the resonant frequency, andwherein the second transmit signal causes the sensing unit to operate asa variable inductor in the second mode when the second frequency isequal to the resonant frequency.
 16. The method of claim 11, wherein thecapacitance is a mutual capacitance formed between the first and secondelectrodes.
 17. The method of claim 11, wherein the first transmitsignal and the second transmit signal are pulse width modulation (PWM)signals.
 18. The method of claim 11, further comprising: generating afirst digital value based on the first receive signal, wherein the firstdigital value is indicative of the capacitance of the sensing unit; andgenerating a second digital value based on the second receive signal,wherein the second digital value is indicative of the inductance of thesensing unit.
 19. The method of claim 11, further comprising:configuring a charge measurement circuit of the sensing circuit tooperate in a first configuration in the first mode, the chargemeasurement circuit coupled to the second terminal; and configuring thecharge measurement circuit to operate in a second configuration in thesecond mode.
 20. A system comprising: a sensing unit comprising aresonant circuit, a first electrode, and a second electrode; and asensing circuit comprising: a first terminal coupled to the firstelectrode via the resonant circuit; a second terminal coupled to thesecond electrode; a signal generator coupled to the first terminal, thesignal generator to generate a first transmit signal in a first mode anda second transmit signal in a second mode; and a receiver channelcoupled to the second terminal, the receiver channel to receive a firstreceive signal in the first mode and a second receive signal in thesecond mode, wherein: the first transmit signal has a first frequencythat is less than or greater than a resonant frequency of the resonantcircuit; the second transmit signal has a second frequency that is equalto the resonant frequency; the first receive signal is indicative of avariable capacitance in the first mode; and the second receive signal isindicative of a variable inductance in the second mode.
 21. The systemof claim 20, wherein the resonant circuit comprises an inductive coil,wherein the first electrode is coupled to a first side of the inductivecoil, and a second side of the inductive coil is coupled to the firstterminal.